1. Technical Field
This invention relates to antenna tracking feed systems, and more particularly, to a traveling wave coupler used in conjunction with a multimode feed horn. 2. Background
Crosslink and downlink point-to-point satellite communications require narrow beamwidth to obtain high antenna gain. In order to maintain reliable communications, a satellite antenna must be pointed accurately towards a signal source. To achieve accurate pointing, satellites commonly employ autotracking systems to provide tracking signals related to pointing errors in elevation and azimuth. The tracking signals control a feedback servoloop of the satellite to orient the satellite as required to position the antenna accurately towards the signal source.
Conventional satellite autotracking systems utilize a monopulse-tracking configuration in which a plurality of antennas, feeding a reflector system, develop three tracking signals, namely an azimuth error signal, an elevation error signal, and a sum signal, which are related to pointing accuracy of the satellite antenna. Monopulse tracking systems are well-known and are described in Radar Handbook by M. I. Skolnik, Second Edition, McGraw-Hill (1990), hereby incorporated by reference.
Conventional autotracking systems use a single multimode feedhorn in conjunction with a mode coupler. The multimode feedhorn is designed to support multiple circular waveguide modes. A fundamental circular TE.sub.11 mode carries a sum radiation pattern used to generate a sum signal and higher order modes, such as TM.sub.01, TE.sub.21 and TE.sub.01, carry a difference radiation pattern used to generate error signals. The mode coupler separates the higher modes from the fundamental modes and thus separate sum and error signals.
The mode coupler used in the conventional single horn tracking system can be an E-plane folded magic tee (MT), a turnstile junction (TJ), or a traveling wave coupler (TWC). The MT approach is a relatively simple way to extract TM.sub.01 mode. The MT approach, however, can not be used for tracking a circularly polarized source because the sum channel responds to linearly polarized signals only. The TJ approach can be used for tracking a circularly polarized source. However, the TJ has complex construction and a large cross-section. Most importantly, the TJ has relatively narrow bandwidth, usually less than 2%. Consequently, the TJ's require tight (high cost) manufacturing tolerance and are sensitive to environmental changes.
The TWC is the only viable approach for wideband operation with circularly polarized fields. Conventional TWCs typically include four or eight arms depending upon whether the source is linearly or circularly polarized, respectively. Each of the coupling arms of the prior art TWC must be balanced to provide an accurate error signal. Amplitude or phase imbalance between coupling arms leads to higher autotracking errors and thus poor aperture efficiency, because any imbalance will result in a null shift in the difference pattern, causing the peak of the sum pattern to be misaligned with the null of the difference pattern. In addition, the multiple arms significantly increase weight of the feed system particularly when the signal source is circularly polarized.
Thus, it would be desirable to provide a TWC at lower cost by simplifying the TWC construction and by reducing or eliminating the need for balancing multiple arms. Further, it would be desirable to provide enhanced performance of a tracking system by eliminating any possible amplitude and phase imbalance. Furthermore, it would be desirable to provide fewer coupling arms in order to make the TWC more compact in the transverse direction to reduce mechanical interference with other mechanical structures, to decrease weight thereof, to simplify the structure and reduce the construction cost.